[enriquep]

So what you actually want is to translate the document. Well... that is a different story, and you should look somewhere else: Calibre has nothing to do with this. I suggest that you try searching in Google for a specialized tool... Or probably that will not be strictly necessary if the document is not too long and complex and if you're willing to do some post-translation correcting and editing... that will be necessary in any case.

Your idea of using an online translator is not bad, but the thing is mainly what you do before that, as online translators usually translate only webpages or plain text... but check in Google if there is any free service that translates pdfs; not that I know of.

IMHO the most adequate approach will in great part depend on the size and structure of the document: for example, there might be some apps that specialize in translating pdfs (I don't know, but Google does), that might be necessary for those long and complex files; but for very short and simple documents a direct copy-and-paste strategy with Google translate (and some patience) could be enough. You can also convert it to TXT (yes: with calibre, in this case) and pass the TXT to Google translate. Also, if the document is available online Google will directly translate it for you... et cetera.

Of course you can also use any of these strategies with any other on- or offline translating service (it doesn't have to be Google translate).

In short: There are several possible solutions - but calibre has (almost) nothing to do with them.